The email address in question may have something weird happening in their other folder structure that is preventing SM from reading/parsing the list of folders before or after adding the new one. I mention this because you note it's only happening to one user.
We have seen issues with folders that have leading blank spaces (attempts to keep them positioned at the top) and so forth.
We had a user that created folders in their Outlook using a leading blank space (or multiples) to organize their folders - which worked, but was very slow to synchronize. We never saw there was a problem until the user tried using their webmail and the folders in question were missing from views. The user wound up having to delete them using Outlook, and recreate with different names before webmail could see them. The data was there in the filesystem on the server, but somewhere the parsing logic couldn't handle the blank spaces in the names and SmarterMail could only "partially" see/work with them.
MailEnable survivor / convert --